Thin film transistors (TFTs), particularly organic TFTs (OTFTs), have drawn increasing attention due to their key role in enabling the next-generation flexible electronics. Low-voltage operation is a highly desirable characteristic of TFTs as it leads to low power consumption. High-capacitance gate insulators are crucial for the achievement of lower operating voltages. This can be seen from the equation governing transistor drain current in the linear regime:ID=μCi(W/L)(VG−VT)VD  (0.1)where μ is the carrier mobility, Ci is the specific areal capacitance of the gate insulator, W/L is the width to length ratio, and VG, VT and VD are the gate, threshold, and drain voltages. With identical device geometry and semiconducting material, an increase in Ci would allow a TFT to output equivalent current with a reduced operation voltage. Inorganic and organic high-k and/or ultrathin dielectrics have achieved a Ci on the order of 0.1 μF cm−2. Alternatively, electrically insulating but ionically conducting solid-state electrolytes (SSEs) have been utilized as gate dielectrics in TFTs, yielding electrolyte-gated TFTs (EGTs). The primary benefits of employing this class of materials is their exceptionally high Ci (1-10 μF cm−2), enabling the operation at <2 V. When a voltage is applied, the ions in the SSE insulators form electrical double layer (EDL) at the interfaces, which can be considered nanometer-thick capacitors, giving rise to the ultrahigh capacitance. Additionally, SSE insulators can be deposited at room temperature, usually by solution-based techniques, allowing them to work with unconventional substrates such as plastic. To compare, the preparation process of some conventional high-k metal oxide dielectrics can be destructive to the substrates required by soft electronics. Among the SSEs applied in TFTs, ionic liquid (IL) gels, here referred to as ion-gels, are particularly promising because of their stability, non-volatility, and high ionic conductivity.
Besides low voltage, high switching speed is another greatly desirable characteristic of modern TFTs. For some demanding applications, such as high-frequency radio-frequency identification (HF RFID) and high-resolution video displays with integrated row and column drivers, megahertz (MHz) operation is required. Moreover, as the direction for the field of organic electronics move beyond demonstration of individual devices to integrated circuits, maximal operating frequency with minimal supply bias is a much sought-after merit for a range of envisioned functions. A high-speed integrated circuit, which consists of multiple stages and exhibit accumulative time delays from each stage, would require the fast-response of every unit. Unfortunately, ion-gel-gated TFTs (denoted hereafter as IGTs) are known to show relatively low switching speed, originated from its slow polarization speed. Normally, transistor switching times depend on the channel length and the charge-carrier mobility. For IGTs, however, the electrolyte polarization speed is often bottlenecking the device's switching speed, hindering the device from fully exploiting the high charge-carrier mobility of the semiconductors. The polarization of ion-gels involves the transport of ions, and therefore is substantially slower than the dielectric polarization mechanism for traditional gate insulators. The time constant of the EDL formation τEDL can be expressed in the following equation:
                              τ          EDL                =                                                                    ⁢                          d              ⁢                                                          ⁢                              ɛɛ                0                                                          2            ⁢                          λ              D                        ⁢            σ                                              (        0.2        )            where d is the gate insulator thickness, ε is the relative permittivity of the electrolyte, ε0 is the vacuum permittivity, λD is the Debye length, σ is the ionic conductivity. Research has been dedicated to reduce τEDL primarily by maximizing the ionic conductivity. Significant progress has been made, allowing the state-of-the-art ion-gels to maintain a high Ci of 1 μF cm−2 up to 10-100 kHz. Although it is recognized that thinning down d is another key route to further improved Ci, a functional ion-gel under 1 μm thick has not been demonstrated yet, probably because of the difficulties to ensure film uniformity and absence of pinholes at this thinness. As a result, beyond 10-100 kHz, the Ci of previously reported ion-gels deteriorates rapidly to several orders of magnitude below 1 μF cm−2. Despite that IGT is theoretically promising to operate with low-voltage in MHz regime, an electrolyte insulator with Ci of 1 μF cm−2 at 1 MHz has never been practically demonstrated, not to mention a MHz IGT device. In our opinion, the lack of nanoscale pinhole-free ion-gels, whose fabrication is compatible with the multi-step transistor production, is hindering the development of IGTs, which urges an immediate solution.
There exists a need for low-voltage MHz IGT devices, and methods of fabricating them.